1. Field of the Invention
The present invention relates to a power or regulating transformer, with a voltage regulator, in a power generation, transmission or distribution system with a rated power ranging from a few hundred kVA up to more than 1000 MVA and with a rated voltage ranging from 3–4 kV and up to very high transmission voltages, 400 kV to 800 kV or higher.
More specifically the invention relates to regulating windings in the power or regulating transformer.
2. Discussion of the Background
The primary task of a power transformer is to act as an electric “gear box”, allowing electric energy to flow from one electrical system to another. The electrical systems interconnected with a transformer usually have different voltages but the same frequency. The power transformer, in its simplest form, has only two types of windings, a primary winding and a secondary winding. The transformation ratio is thereby fixed, that is, not possible to regulate. However, there is a need to be able to control the active and reactive power flow between the electrical systems in order to run systems in an efficient mariner or, which is more fundamental, maintain the stability of the systems. Therefore, a regulator, having a regulating winding or windings, are often incorporated in the power transformer, or in a separate regulating transformer connected in series with the power transformer. These regulating transformers are sometimes referred to as “booster transformers”. In order to obtain a flexible control over the electrical systems, the active and reactive power flow between the systems is preferably controlled independently of each other. In order to achieve this, the phase-shift between the phase voltages of the systems must be able to be controlled with a variable angle.
In E. Wirth, J-F. Ravot: “Regulation transformers in power systems—new concepts and applications”, ABB Review 4/1997, pp. 12–20, a three-phase power transformer with integrated regulator is described. The transformer has three core legs, each leg associated with phases U, V and W respectively. Around each leg a primary winding, a secondary winding, an in-phase control winding, a first quadrature control winding and a second quadrature control winding are wound. The in-phase control winding, the first quadrature control winding and the second quadrature control winding are all regulating windings. The in-phase control winding of each phase is, via a first tap changer common for all three phases, connected in series with the secondary winding of the same phase. This makes it possible to regulate the amplitude of the secondary voltage of each phase, that is, obtain in-phase regulation between the primary and secondary of the transformer. Via a second tap changer, also common for all three phases, the primary winding of phase U is connected in series with the first quadrature control winding of phase V and the second quadrature control winding of phase W. In a similar fashion the primary winding of phase V is connected in series with the quadrature control windings of phases W and U, and the primary winding of phase W is connected in series with the quadrature control windings of phases U and V. The arrangement of the quadrature control windings and the second tap changer is such, that the voltage across the series connected quadrature windings always is perpendicular to the voltage across the primary winding. The requirements for so called quadrature regulation is therefore fulfilled. The phase angle of the primary voltage of each phase can thus be regulated by means of the second tap changer. By combining the in-phase regulation on the secondary side with the quadrature regulation on the primary side of the transformer, it is possible to, for each phase, phase shift the voltage across the transformer and thus accomplish an independent active and reactive power flow control between the primary and secondary side of the transformer.
In N. Mohan: “MPTC: An economical alternative to universal power flow controllers”, EPE 97 in Trondheim, pp. 3.1027–3.1032, another regulation system to control the power flow is described. It is made of a three phase regulating transformer where fractions of the primary voltages, by way of secondary windings and via thyristor bridge arrangements, are injected in series with each phase voltage. The secondary windings thus act as regulation windings. By controlling the switches in the thyristor bridges, a linear combination of the voltages over the secondary windings can be added to each phase voltage and each phase voltage can thus be phase-shifted by a variable angle.
The regulator of the three phase transformer by E. Wirth, J-F. Ravot described above, is made of nine regulating windings, three per phase, and two tap changers. The regulator of the regulating transformer by N. Mohan, is made of one regulating winding and one thyristor bridge per phase, totaling three regulating windings and three thyristor bridges. Depending on the regulating voltage, the space occupied by the regulating windings may be quite large due to insulation requirements.